Waveguide Grating Device

ABSTRACT

An optical waveguide comprises at least two TIR surface and contains a grating. Input TIR light with a first angular range along a first propagation direction undergoes at least two diffractions at the grating. Each diffraction directs light into a unique TIR angular range along a second propagation direction.

CROSS REFERENCE TO PRIORITY APPLICATION

This application is a continuation of U.S. application Ser. No.17/457,893 filed Dec. 6, 2021, which is a continuation of U.S.application Ser. No. 16/734,208 filed Jan. 3, 2020, issued on Dec. 7,2021 as U.S. Pat. No. 11,194,098, which application is a continuation ofU.S. application Ser. No. 16/178,104 filed Nov. 1, 2018, issued on Jan.7, 2020 as U.S. Pat. No. 10,527,797, which application is a continuationof U.S. application Ser. No. 15/807,149 filed Nov. 8, 2017, issued onDec. 18, 2018 as U.S. Pat. No. 10,156,681, which is a continuation ofU.S. application Ser. No. 15/468,536 filed Mar. 24, 2017, issued on Nov.21, 2017 as U.S. Pat. No. 9,823,423, which is a continuation of U.S.application Ser. No. 14/620,969 filed Feb. 12, 2015, issued on Apr. 25,2017 as U.S. Pat. No. 9,632,226, the disclosures of which areincorporated herein by reference in their entireties.

BACKGROUND OF THE INVENTION

This invention relates to a waveguide device, and more particularly to awaveguide holographic grating. Waveguide optics is currently beingconsidered for a range of display and sensor applications for which theability of waveguides to integrate multiple optical functions into athin, transparent, lightweight substrate is of key importance. This newapproach is stimulating new product developments including near-eyedisplays for Augmented Reality (AR) and Virtual Reality (VR), compactHeads Up Display (HUDs) for aviation and road transport and sensors forbiometric and laser radar (LIDAR) applications. Waveguides are limitedin terms of the range of ray angles that can be efficiently guided witha substrate. One solution addressed in the above references is to useholographic gratings for in-coupling and out-coupling light. However,while transmission holographic gratings perform these functionsefficiently, their narrow angular bandwidth imposes even tighter angularlimits on the image content that can be transmitted down a waveguide.Using the teachings contained in the above references it is possible toovercome these angular limitations by stacking or multiplexing gratings.Stacking is currently limited by holographic scatter while the number ofgratings that can be multiplexed in a single waveguide is limited bycurrent material modulation uniformity. One potentially very useful typeof grating, called a fold grating, is unique in allowing changes in beampropagation direction and beam expansion to be accomplished in a singlegrating layer. However, prototype fold gratings have been found to havenarrow angular bandwidths. There is therefore a need for a waveguidefold grating with an angular bandwidth that addresses the full angularcapability of a waveguide.

SUMMARY OF THE INVENTION

It is a first object of the invention to provide a waveguide foldgrating with an angular bandwidth that addresses the full angularcapability of a waveguide.

The object of the invention is achieved in first embodiment of theinvention in which there is provided an optical waveguide with least twoTIR surfaces containing a grating. Input TIR light with a first angularrange along a first propagation direction undergoes at least twodiffractions, wherein each ray from the first angular range and itscorresponding diffracted ray lie on the diffraction cone of the grating,wherein each diffraction provides a unique TIR angular range along asecond propagation direction.

In one embodiment each ray from the first angular range and itscorresponding diffracted ray are offset from the k-vector of the gratingby an angle less than an angle at which the diffraction efficiency is apredefined fraction of the peak diffraction efficiency.

In one embodiment each unique TIR angular range provides a uniquediffraction efficiency versus angle characteristic. In one embodimentthe diffraction efficiency versus angle characteristics do not overlap.In one embodiment the diffraction efficiency versus anglecharacteristics overlap.

In one embodiment the angular separation of the diffracted ray vectorsproduced in the two diffractions is equal to the diffraction cone angle.

In one embodiment the grating is a leaky grating providing amultiplicity of diffractions, wherein only two diffractions arecharacterized by a unique pair of incident and diffracted ray vectors onthe diffraction cone.

In one embodiment the grating is a Bragg grating or a SBG and isrecorded in one of a HPDLC grating, uniform modulation grating orreverse mode HPDLC grating.

In one embodiment the diffracted light has a polarization state producedby aligning the average relative permittivity tensor of the grating, thepolarization state being one of linearly, elliptically or randomlypolarized.

In one embodiment non-diffracted light has a polarization state producedby aligning the average relative permittivity tensor of the grating, thepolarization state being one of linearly, elliptically or randomlypolarized.

In one embodiment the grating is one of a multiplexed set of gratings.

In one embodiment the grating has a spatially varying thickness.

In one embodiment the grating has spatially-varying diffractionefficiency.

In one embodiment the grating has spatially-varying k-vector directions.

In one embodiment the grating comprises an array of selectivelyswitchable elements.

In one embodiment the diffracted light is transmitted through a TIR faceof the waveguide.

In one embodiment the apparatus further comprises at least one of awaveguide input coupler for inputting light through a face of thewaveguide and directing it into the first propagation path, and awaveguide output coupler for outputting the diffracted light through aface of the waveguide, wherein each of the input and output couplers isone of a grating or prism.

In one embodiment at least one of the waveguide input coupler and thewaveguide output coupler is a grating configured such that gratingreciprocity is satisfied within the waveguide.

In one embodiment the input light is modulated with temporally-varyingangularly-distributed information content.

In one embodiment the waveguide has first and second parallel TIRsurfaces, the grating diffracting light out of the first propagationdirection into a second propagation direction, the grating characterizedin that a portion of light reflected from the first TIR surface isdiffracted into TIR along the second propagation direction in a firstTIR angular range and a portion of light reflected from the second TIRsurface is diffracted into TIR along the second propagation direction ina TIR range.

In one embodiment the first and second propagation direction areorthogonally disposed in the plane of the waveguide.

In one embodiment the apparatus further comprises a second gratingoverlaying the first grating. The second grating deflecting light in thefirst propagation direction into a second propagation direction withinthe waveguide, the second grating characterized in that a portion oflight reflected from the first TIR surface is diffracted into TIR alongthe second propagation direction in a third TIR angular range and aportion of light reflected from the second TIR surface is diffractedinto TIR along the second propagation direction in a fourth TIR angularrange. In one embodiment the first and second gratings are multiplexed.In one embodiment each the third and fourth TIR angular rangescorrespond to unique diffraction efficiency versus anglecharacteristics.

In one embodiment input TIR light width an angular range in a thirdpropagation direction undergoes at least one diffraction along a uniquevector on the diffraction cone of the grating. In one embodiment thefirst and the third propagation direction are in opposing directions. Inone embodiment the TIR angular range of the input TIR light in the thirdpropagation direction does not overlap with the diffraction efficiencyversus angle characteristics of the light in the second propagationdirection. In one embodiment the input TIR light in the firstpropagation direction and the input TIR light in third secondpropagation direction have different wavelengths. In one embodiment thegrating multiplexes first and second gratings. Input TIR light in thefirst propagation direction is diffracted by the first multiplexedgrating and input TIR light in the third propagation direction isdiffracted by the second multiplexed grating. In one embodiment theapparatus further comprises a second grating for diffracting input TIRlight travelling along the third propagation direction.

A more complete understanding of the invention can be obtained byconsidering the following detailed description in conjunction with theaccompanying drawings, wherein like index numerals indicate like parts.For purposes of clarity, details relating to technical material that isknown in the technical fields related to the invention have not beendescribed in detail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of the optical geometry of a generalwaveguide grating in one embodiment.

FIG. 2 is a schematic plan view illustrating the principles of a foldgrating.

FIG. 3A is schematic illustration of a waveguide grating showing a firstaspect of light diffraction in one embodiment.

FIG. 3B is schematic illustration of a waveguide grating showing asecond aspect of light diffraction in one embodiment.

FIG. 4A is schematic illustration of a waveguide with slanted gratingfringes showing a first aspect of light diffraction in one embodiment.

FIG. 4B is schematic illustration of a waveguide with slanted gratingfringes showing a second aspect of light diffraction in one embodiment.

FIG. 5 is a plot of the diffraction efficiency versus anglecharacteristic of a waveguide grating based on the embodiment of FIGS.4A-4B.

FIG. 6 is schematic illustration of a waveguide grating in oneembodiment.

FIG. 7 is a plot showing the diffraction efficiency versus anglecharacteristics of a waveguide grating used in the embodiment of FIG. 6.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be further described by way of example only withreference to the accompanying drawings. It will apparent to thoseskilled in the art that the present invention may be practiced with someor all of the present invention as disclosed in the followingdescription. For the purposes of explaining the invention well-knownfeatures of optical technology known to those skilled in the art ofoptical design and visual displays have been omitted or simplified inorder not to obscure the basic principles of the invention. Unlessotherwise stated the term “on-axis” in relation to a ray or a beamdirection refers to propagation parallel to an axis normal to thesurfaces of the optical components described in relation to theinvention. In the following description the terms light, ray, beam anddirection may be used interchangeably and in association with each otherto indicate the direction of propagation of light energy alongrectilinear trajectories. Parts of the following description will bepresented using terminology commonly employed by those skilled in theart of optical design. It should also be noted that in the followingdescription of the invention repeated usage of the phrase “in oneembodiment” does not necessarily refer to the same embodiment.

FIG. 1 is a schematic illustration of a waveguide device according tothe principles of the invention comprising: a waveguide having at leasttwo total internal reflection (TIR) surfaces 10, 11 containing at leastone grating 12. The TIR surfaces may be angled with respect to eachother. The TIR surfaces will normally be planar. In some embodiments theTIR surfaces may be curved in one or two orthogonal directions. Thegrating has a k vector 1000, where a k-vector is conventionally definedas the unit vector normal to the grating fringe surfaces. A waveguideinput coupler, which is not illustrated, couples input light 1001 with amultiplicity input ray angles as represented by the rays 1002-1005 intothe waveguide. The waveguide input coupler may be a grating or prism.The invention does not assume any particular coupling means.

The ray 1004, which is the principal ray of a ray bundle covering afirst angular range, is coupled in the TIR path 1008 in the firstpropagation direction 1006. The TIR path 1008 contains downward-goingray segments such as 1009 and upward-going ray segments such as 1010.For ease of explanation the ray segments 1009 and 1010 are also labelledby the vectors rdown and rup. The invention may be applied to a range ofwaveguide geometries in which tilted planar surfaces provide TIR.However, for the purpose of explaining the invention we invite thereader to visualize FIG. 1 as representing the simplest waveguidecovered by the invention; that is, one with two parallel TIR surface.All TIR in this case takes place at these two surfaces. In the followingdescription downward-going refers to TIR rays reflected from the top TIRsurface and upward-going refers to rays reflected from the bottom TIRsurface. In the case of waveguides with more than two TIR surfacescharacterizing the ray paths simply in terms of upward or downwardpropagation is not practical; it is more appropriate to use vectorformalism. (Note that for the purposes of the invention the term“propagation direction” refers to foe general direction of energytransfer of the TIR beams and not to the individual up-going anddownward-going ray paths described above.) Referring again to FIG. 1another TIR path having a different TIR angle (defined as the anglebetween the ray and the normal to the TIR surface) results from theinput ray 1005, the principal ray of a ray bundle covering a secondangular range, which is coupled into the TIR path 1011 containupward-going ray segments such as 1012 and downward going ray segmentssuch as 1013. The ray segments 1012 and 1013 are also labelled by thevectors r'up and r'down. The two TIR paths are deflected into a secondpropagation direction 1007. Hence the input light is diffracted leasttwo times by the grating (each diffraction corresponding to a uniqueangular range). Only rays that exactly satisfy the Bragg equation willbe diffracted with high efficiency. (Note that in the case where thegrating is a leaky grating ie one in which a small amount of light isdiffracted at each bounce only two bounces will give rise to a uniquepair of incident and diffracted rays on the diffraction cone.) Incidentrays and diffracted rays satisfying the Bragg condition lie on a surfaceapproximating to a cone. In one particular case of interest the angularseparation of the diffracted ray vectors produced in the twodiffractions is equal to the diffraction cone angle. Rays not meetingthe Bragg condition will have progressively lower diffraction efficiencywith increasing angular (and wavelength) deviation from the on-Braggangle (and wavelength) with the limiting condition typically beingdefined as 50% of the peak efficiency. Each of the diffracted ray pathscorresponds to the peak efficiency ray of a unique range of diffractedray angles. To ensure that most of the light is diffracted with highefficiency each input ray and its corresponding diffracted ray areoffset from the diffraction cone of the grating by an angle less thanhalf the diffraction efficiency angular bandwidth. The latter isfrequently defined as the angle range over which the diffractionefficiency is greater than or equal to 50% of the peak efficiency.However, other measures of the limiting diffraction efficiency may beused in the present invention depending on factors such as the requiredlight output uniformity. As we will see later, in some embodiments wherethe waveguide grating is used in a sensor it is advantageous to have nonoverlapping angular characteristics in order to separated illuminationand signal light.

By configuring the grating to diffract upward-going and downward-goingrays the angular range over which the grating operates is greatlyexpanded. This will be illustrated in the case of one particular foldgrating design later in the description. In one embodiment each range ofdiffracted angles corresponding to the range of input rays havingprincipal rays 1008, 1011 results a unique diffraction efficiency versusangle characteristic, where the angle referred to is that betweenincident or diffracted ray and the k-vector in the medium (glass orplastic). In most practical applications of the invention the angularmeasure of interest is the angular bandwidth in air. For example in thecase of a waveguide display it is useful to specify the angularbandwidth of the display as observed from the eye box (or exit pupil).This or other measures of the angular bandwidth can be determined usingbasic geometrical optics. The diffraction efficiency versus anglecharacteristics may be engineered to overlap with the degree of overlapdepending on the application. Where high uniformity is required a fairlyhigh degree of overlap is required to remove effects such as banding.Later in the description we will describe how the waveguide grating maybe used to provide more than one optical path through a waveguide (forexample in an optical receiver/transmitter). In such applications it isdesirable to keep the overlap between the diffraction efficiency versusangle characteristics to a minimum to avoid crosstalk between thereceive and transmit channels.

The grating used in the invention is desirably a Bragg grating (alsoreferred to as a volume grating). Bragg gratings have high efficiencywith little light being diffracted into higher orders. The relativeamount of light in the diffracted and zero order can be varied bycontrolling the refractive index modulation of the grating, a propertywhich is used to make lossy waveguide gratings for extracting light overa large pupil.

One important class of gratings is known as Switchable Bragg Gratings(SBG). SBGs are fabricated by first placing a thin film of a mixture ofphotopolymerizable monomers and liquid crystal material between parallelglass plates. One or both glass plates support electrodes, typicallytransparent indium tin oxide films, for applying an electric fieldacross the film. A volume phase grating is then recorded by illuminatingthe liquid material (often referred to as the syrup) with two mutuallycoherent laser beams, which interfere to form a slanted fringe gratingstructure. During the recording process, the monomers polymerize and themixture undergoes a phase separation, creating regions densely populatedby liquid crystal micro-droplets, interspersed with regions of clearpolymer. The alternating liquid crystal-rich and liquid crystal-depletedregions form the fringe planes of the grating. The resulting volumephase grating can exhibit very high diffraction efficiency, which may becontrolled by the magnitude of the electric field applied across thefilm. When an electric field is applied to the grating via transparentelectrodes, the natural orientation of the LC droplets is changedcausing the refractive index modulation of the fringes to reduce and thehologram diffraction efficiency to drop to very low levels. Typically,SBG Elements are switched clear in 30 μs, with a longer relaxation timeto switch ON. Note that the diffraction efficiency of the device can beadjusted, by means of the applied voltage, over a continuous range. Thedevice exhibits near 100% efficiency with no voltage applied andessentially zero efficiency with a sufficiently high voltage applied. Incertain types of HPDLC devices magnetic fields may be used to controlthe LC orientation. In certain types of HPDLC phase separation of the LCmaterial from the polymer may be accomplished to such a degree that nodiscernible droplet structure results. A SBG may also be used as apassive grating. In this mode its chief benefit is a uniquely highrefractive index modulation.

SBGs may be used to provide transmission or reflection gratings for freespace applications. SBGs may be implemented as waveguide devices inwhich the HPDLC forms either the waveguide core or an evanescentlycoupled layer in proximity to the waveguide. The parallel glass platesused to form the HPDLC cell provide a total internal reflection (TIR)light guiding structure. Light is coupled out of the SBG when theswitchable grating diffracts the light at an angle beyond the TIRcondition. Waveguides are currently of interest in a range of displayand sensor applications. Although much of the earlier work on HPDLC hasbeen directed at reflection holograms transmission devices are provingto be much more versatile as optical system building blocks. Typically,the HPDLC used in SBGs comprise liquid crystal (LC), monomers,photoinitiator dyes, and coinitiators. The mixture frequently includes asurfactant. The patent and scientific literature contains many examplesof material systems and processes that may be used to fabricate SBGs.Two fundamental patents are: U.S. Pat. No. 5,942,157 by Sutherland, andU.S. Pat. No. 5,751,452 by Tanaka et al. Both filings describe monomerand liquid crystal material combinations suitable for fabricating SBGdevices.

One of the known attributes of transmission SBGs is that the LCmolecules tend to align normal to the grating fringe planes. The effectof the LC molecule alignment is that transmission SBGs efficientlydiffract P polarized light (ie light with the polarization vector in theplane of incidence) but have nearly zero diffraction efficiency for Spolarized light (ie light with the polarization vector normal to theplane of incidence. Transmission SBGs may not be used at near-grazingincidence as the diffraction efficiency of any grating for Ppolarization falls to zero when the included angle between the incidentand reflected light is small.

In one embodiment the gratings are recorded in uniform modulation liquidcrystal-polymer material system such as the ones disclosed in UnitedState Patent Application Publication No.: US2007/0019152 by Caputo et aland PCT Application No.: PCT/EP2005/006950 by Stumpe et al. both ofwhich are incorporated herein by reference in their entireties. Uniformmodulation gratings are characterized by high refractive indexmodulation (and hence high diffraction efficiency) and low scatter. Inone embodiment the gratings are recorded in a reverse mode HPDLCmaterial. Reverse mode HPDLC differs from conventional HPDLC in that thegrating is passive when no electric field is applied and becomesdiffractive in the presence of an electric field. The reverse mode HPDLCmay be based on any of the recipes and processes disclosed in PCTApplication No.: PCT/GB2012/000680, entitled IMPROVEMENTS TO HOLOGRAPHICPOLYMER DISPERSED LIQUID CRYSTAL MATERIALS AND DEVICES. The grating maybe recorded in any of the above material systems but used in a passive(non-switching) mode. The fabrication process is identical to that usedfor switched but with the electrode coating stage being omitted. LCpolymer material systems are highly desirable in view of their highindex modulation.

In a birefringent grating the index has two components: extraordinary(ne) and ordinary (no) indices. The extraordinary index is defined bythe optic axis (ie axis of symmetry) of a uniaxial crystal as determinedby the average LC director direction. The ordinary index corresponds tothe other two orthogonal axes. More generally the index is characterisedusing a permittivity tensor. To the best of the inventors' knowledge theoptic axis in LC-based gratings tends to align normal to the Braggfringes ie along the K-vectors. For reasonably small grating slantangles applying an electric field across the cell re-orients thedirectors normal to the waveguide faces, effectively clearing thegrating. An incident ray sees an effective index dependent on both theextraordinary and ordinary indices with the result that the Poyntingvector and wave vector are separated by a small angle. This effectbecomes more pronounced at higher angles. In one embodiment thediffracted rays have a polarization state produced by aligning theaverage relative permittivity tensor of the grating. It is also usefulto have the capability of controlling the polarization of non-diffractedlight. Accordingly, in one embodiment the non-diffracted rays have apolarization state produced by aligning the average relativepermittivity tensor of the grating. The polarization states may be oneof randomly, linearly or elliptically polarized. In applications wherethe diffracted light interacts with another grating is desirable that itis linearly polarized. For example SBGs have highest diffractionefficiency for P-polarized light. In a waveguide the birefringence ofthe LC will tend to rotate the polarization of the light at each TIRbounce. This has the effect of scrambling the polarization of the light.Initial experiments point to the light not becoming fully randomlypolarized. However, this is likely to depend on the characteristics ofthe birefringence. In one embodiment the permittivity tensor is modifiedto provide a random polarization state at the output end of the grating.Random polarization is desirable in applications in which the diffractedlight is viewed directly, for example in a display.

In one embodiment the grating is one of a multiplexed set of gratings.Each grating may operate over a defined angular or spectral range.Multiplexing allows the angular bandwidth and color space to be expandedwithout significantly increasing the number of waveguide layers. In oneembodiment the grating has a spatially varying thickness. Sincediffraction efficiency is proportional to the grating thickness whileangular bandwidth is inversely propagation to grating thickness allowingthe uniformity of the diffracted light to be controlled. In oneembodiment the grating has spatially-varying k-vector directions forcontrolling the efficiency, uniformity and angular range of the grating.In one embodiment grating has spatially-varying diffraction efficiency.The application of multiplexing, and spatial varying thickness, k-vectordirections and diffraction efficiency in the present invention is basedon the embodiments, drawings and teachings provided in U.S. patentapplication Ser. No. 13/506,389 entitled COMPACT EDGE ILLUMINATEDDIFFRACTIVE DISPLAY, U.S. Pat. No. 8,233,204 entitled OPTICAL DISPLAYS,PCT Application No.: PCT/US2006/043938, entitled METHOD AND APPARATUSFOR PROVIDING A TRANSPARENT DISPLAY, PCT Application No.PCT/GB2012/000677 entitled WEARABLE DATA DISPLAY, U.S. patentapplication Ser. No. 13/317,468 entitled COMPACT EDGE ILLUMINATEDEYEGLASS DISPLAY, U.S. patent application Ser. No. 13/869,866 entitledHOLOGRAPHIC WIDE ANGLE DISPLAY, and U.S. patent application Ser. No.13/844,456 entitled TRANSPARENT WAVEGUIDE DISPLAY.

The diffracted light may be transmitted through a face of the waveguide.In a waveguide display this light would be viewed directly by the user.In other embodiments the diffracted light may continue to undergo TIR inthe waveguide. For example it may interact with an output grating whichprovides beam expansion and diffracts the light out of the waveguide.This arrangement may be used in a waveguide display based on theprinciples disclosed in the above references. In one embodiment thediffracted light may be directed out of the waveguide using a prism.

In one embodiment the waveguide further comprises at least one of awaveguide input coupler for inputting light through a face of thewaveguide and directing it into the first propagation path, and awaveguide output coupler for outputting the diffracted light through aface of the waveguide, wherein each of the input and output couplers isone of a grating or prism. In one embodiment at least one of thewaveguide input coupler and the waveguide output coupler is a gratingconfigured such that grating reciprocity is satisfied within thewaveguide.

In one embodiment the grating is fold grating used for changing beamdirection and providing beam expansion within a waveguide. Thisconfiguration would typically be use in a waveguide display of the typedisclosed in the reference patent applications. Gratings designed forcoupling light into or out of a waveguide are tilted around an axislying in the waveguide plane. Fold gratings have a more generalizedtilt. In their simplest implementation, as used in the presentinvention, they are tilted around an axis perpendicular to the waveguideplane such they deflect beams in the waveguide plane. More generally,they may have tills defined by two rotation angles so that, for example,light can be coupled into the waveguide and deflected into an orthogonaldirection inside the waveguide, all in one step. Of particularimportance for the present invention, they can perform ninety degreeturning and two-axis beam expansion in a single layer, avoidingorthogonal superimposed grating layers. FIG. 2 is a plan view of thefold grating 22. When the set of rays 2015 encounter the grating, theydiffract in a manner that changes the direction of propagation by 90°.Unlike a conventional vertical extraction grating, the light does notleave the waveguide. Note that when a ray encounters the grating,regardless of whether it intersects the grating from above or below, afraction of it changes direction and the remainder continues unimpeded.A typical ray will interact many times with vertically (in the Ydirection) while some light will be moving laterally (in the Xdirection). From a design perspective, it is desirable to engineer theamount of light 2016 emerging from the output edge of the grating to beuniformly distributed laterally and the amount of light 2017 emergingfrom the side edge of the grating to be as small as possible.

We next consider fold grating architectures using a grating according tothe principles of the invention. In the embodiments shown in FIGS. 3-4the grating contained in a planar waveguide, that is, on with twoopposing TIR surfaces deflects input light in a first propagationdirection into a second propagation direction. As in the embodiment ofFIG. 1 the grating is designed such that portion of the upward-going TIRlight interacting with the grating is diffracted into a first range ofangles and a portion of downward-going TIR light interacting with thegrating is diffracted into a second range of angles. The upward-goingTIR light and downward-going TIR light rays are offset from the k-vectorof the grating by an angle smaller than half the diffraction efficiencyangular bandwidth. Turning first to the embodiment of FIG. 3 , thewaveguide 20 contains grating fringes 21 disposed at ninety degrees tothe waveguide TIR faces and slanted in the waveguide plane, typically by45 degrees to provide 90 degrees beam deflection. In FIG. 3A a first TIRpath lies in the input propagation plane 2001 and, after diffraction inthe output propagation plane 2002. TIR light 2004 in the propagationplane 2001 having a TIR angle 2005 relative to the waveguide planenormal 2006 strikes the grating fringe as an upward-going ray 2007 whichis diffracted into the TIR direction 2008 lying inside the propagationplane 2002. In FIG. 3B a second TIR path in the input propagation plane2001 indicated by 2010 has a TIR angle 2011 relative to the waveguideplane normal 2006 strikes the grating fringe as a downward-going ray2013 which is diffracted into the TIR direction 2014 lying inside thepropagation plane 2002. Since the upward-going and downward-going TIRrays are symmetric in this case there is only one peak in the outputdiffraction efficiency versus angle characteristic.

Turning next to the embodiment of FIG. 4 , we consider a waveguide 30containing grating fringes 31 slanted with respect to the waveguide TIRfaces and relative to the waveguide plane. Again the latter willtypically be 45 degrees to provide 90 degrees beam deflection. In FIG.4A a first TIR path lies in the input propagation plane 2020 and, afterdiffraction in the output propagation plane 2021. The grating has ak-vector 2022 also labelled by the symbol k. The tilt angle 2023 of thegrating fringes relative to the waveguide surface normal 2024 is alsoindicated. TIR light 2025 in the propagation plane 2001 having a TIRangle 2026 relative to the waveguide plane normal 2027 strikes thegrating fringe as an upward-going ray 2028 which is diffracted into theTIR direction 2029 lying inside the propagation plane 2021. In FIG. 4B asecond TIR path in the input propagation plane 2001 indicated by 2030has a TIR angle 2031 relative to the waveguide plane normal 2027 strikesthe grating fringe as a downward-going ray 2033 which is diffracted intothe TIR direction 2034 lying inside the output propagation plane 2021.Since the upward-going and downward-going TIR rays are asymmetric inthis case there are two peaks in the output DE versus anglecharacteristic.

In one embodiment based on the embodiment illustrated in FIG. 4 , asecond grating overlays the first grating. The second grating deflectslight in the first propagation direction into a second propagationdirection within the waveguide. The second grating is characterized inthat a portion of upward-going TIR light interacting with the secondgrating is diffracted into a third range of angles and a portion ofdownward-going TIR light interacting with the grating is diffracted intoa fourth range of angles. The upward-going TIR light and thedownward-going TIR light are offset from the k-vector of the secondgrating by an angle smaller than half the diffraction efficiency angularbandwidth. FIG. 5 is plot of diffraction efficiency versus angle (inwaveguide) for a waveguide containing two stacked gratings based on theembodiment of FIG. 4 . Each grating is configured to provide two uniqueefficiency versus angle characteristics; that is, four in total.

Although the invention is primarily motivated by the need to improve theangular bandwidth of a waveguide display it may also applied to otheroptical devices. In particular it may be applied to sensors such as eyetrackers, LIDAR and biometric scanners. To this end FIGS. 1-4 may referto a sensor waveguide if the directions of all rays illustrated arereversed. The input coupler would become an output coupler for directingsignal light onto a detector. The benefit of the present invention isthat the range of detection handles can be expanded to address the fullangular capability of a waveguide. With regard to eye tracking theinvention may be used in the waveguide eye trackers disclosed inPCT/GB2014/000197 entitled HOLOGRAPHIC WAVEGUIDE EYE TRACKER, U.S.Provisional Patent Application No. 62/071,534 entitled HOLOGRAPHICWAVEGUIDE FOR TRACKING AN OBJECT IN 3D SPACE, PCT/GB2013/000210 entitledAPPARATUS FOR EYE TRACKING, PCT Application No. PCT/GB2013/000210entitled APPARATUS FOR EYE TRACKING.

In one group of embodiments the waveguide grating provides at least twodifferent light paths. As indicated above one important area ofapplication of such embodiments is in the field of sensors. The numberof light paths that can be handled by a grating waveguide will depend onthe available angular bandwidth of the grating. To illustrate the basicprinciple of a waveguide grating providing two different light pathsFIG. 6 shows a single waveguide grating fringe 40 with a k-vector 2050which in turn provides the axis of the diffraction cone 2051. A firstoptical path corresponds to the first and second beam propagationdirection indicated by 2052, 2053. The beams propagation paths are alsolabelled by the encircled numerals 1-4. A second optical path isprovided by the third and fourth beam propagation directions indicatedby 2054,2055. In a typical application the first optical path might be atransmit channel for transporting light from an illumination source(which is coupled to the waveguide) to a reflecting surface outside thewaveguide. The second optical path would provide a receive channel fortransporting reflected light from the external surface to a detector(coupled to the waveguide). To simplify the description the interveninginput/output coupling gratings and other elements commonly used inwaveguides are not considered.

Turning again to FIG. 6 the TIR paths around the first beam propagationdirection are represented by the rays 2056,2057 and TIR baths around thethird beam propagation direction are represented by rays 2058,2059. TheTIR surfaces have not been illustrated. It may be helpful to visualizethe latter as parallel to the plane of the drawing. Hence the beampropagation directions are substantially coplanar. As a further aid tounderstanding the drawings the TIR paths have been rotated by 90 degreesaround the propagation direction vectors.

In FIG. 6 the propagation directions have been illustrated in as generalas possible. In practice the range of directions will be determined bygrating and waveguide angular bandwidth considerations as well as theconstraints on where components such as light sources and detectors maybe located relative to the waveguide. In one embodiment the second andthird beam propagation directions may be in substantially opposingdirections. This principle is used in eye trackers disclosed in thecited references. To avoid crosstalk between the receive and transmitchannels it is important that the diffraction efficiency versus anglecharacteristics for the optical paths do not overlap substantially. Ingeneral to avoid any possible stray light paths the diffractionefficiency versus angle characteristics for all four beam propagationdirections should have little or no overlap. The diffraction efficiencyversus angle plots 2060,2061 for the second and third propagationdirections are shown in FIG. 7 . The invention allows for at least twodiffractions for each of the two light paths. In the case of sensorsboth the receive and transmit channels may use two diffractions.However, in many applications wide angular bandwidth may only berequired in the detection channel. In one embodiment the two opticalpaths may propagate different wavelengths. In one embodiment the twooptical paths may propagate different polarization states. The abilityto provide two or more optical paths through a waveguide may haveapplications in fields such as laser instrumentation, optical computing,robotics and industrial process control and monitoring.

In the case of waveguide displays the input light is modulated withtemporally-varying angularly-distributed information content using aspatial light modulator such as a liquid crystal display panel or usinga laser scanner based on MEMs or other beam deflection technology. Atypical HMD architecture is a waveguide one or more stacked inputgratings for coupling in collimated light from an image generator, onefold grating, and one or more output gratings for output vertically andhorizontally pupil-expanded light towards an eye box form which the fullimage may be viewed.

It should be emphasized that the drawings are exemplary and that thedimensions have been exaggerated. For example thicknesses of the SBGlayers have been greatly exaggerated.

In any of the above embodiments the waveguides may be curved or formedfrom a mosaic of planar or curved facets.

A waveguide device based on any of the above-described embodiments maybe implemented using plastic substrates using the materials andprocesses disclosed in PCT Application No.: PCT/GB2012/000680, entitledIMPROVEMENTS TO HOLOGRAPHIC POLYMER DISPERSED LIQUID CRYSTAL MATERIALSAND DEVICES.

It should be understood by those skilled in the art that while thepresent invention has been described with reference to exemplaryembodiments, it is to be understood that the invention is not limited tothe disclosed exemplary embodiments. Various modifications,combinations, sub-combinations and alterations may occur depending ondesign requirements and other factors insofar as they are within thescope of the appended claims or the equivalents thereof.

What is claimed is:
 1. An optical waveguide comprising: at least two TIR surfaces and containing a grating of a first prescription configured such that an input TIR light with a first angular range along a first propagation direction undergoes at least two diffractions within said grating and undergoes a change in propagation direction from said first propagation direction to a second propagation direction, wherein each ray from said first angular range and its corresponding diffracted rays lie on a diffraction cone of said grating, wherein each diffraction provides a unique diffraction efficiency versus angle characteristic along said second propagation direction, wherein one of said diffractive efficiency versus angle characteristics corresponds to rays that do not meet the condition for TIR at said TIR surfaces.
 2. The optical waveguide of claim 1 wherein said TIR surfaces are parallel.
 3. The optical waveguide of claim 1 wherein said waveguide forms a pupil.
 4. The optical waveguide of claim 1 wherein the condition that one of said diffractive efficiency versus angle characteristics corresponds to rays that do not meet the condition for TIR at said TIR surfaces applies to higher angles of said first angular range at some locations along said second propagation path and applies to lower angles of said first angular range at some locations along said second propagation path.
 5. The optical waveguide of claim 1 wherein said grating is a fold grating.
 6. The optical waveguide of claim 1 wherein said grating extracts light from said waveguide.
 7. The optical waveguide of claim 1 wherein a ray from said first angular range and its corresponding diffracted ray are each offset from said diffraction cone by an angle not exceeding half the diffraction angular bandwidth of said grating.
 8. The optical waveguide of claim 1 wherein said diffraction efficiency versus angle characteristics do not overlap.
 9. The optical waveguide of claim 1 wherein said diffraction efficiency versus angle characteristics overlap.
 10. The optical waveguide of claim 1 wherein the angular separation of the diffracted ray vectors produced in said two diffractions is equal to the diffraction cone angle.
 11. The optical waveguide of claim 1 wherein said grating is a leaky grating providing a multiplicity of diffractions, wherein only two diffractions are characterized by a unique pair of incident and diffracted ray vectors on said diffraction cone.
 12. The optical waveguide of claim 1 wherein said grating is a Bragg grating or a SBG and is recorded in one of a HPDLC grating, uniform modulation grating or reverse mode HPDLC grating.
 13. The optical waveguide of claim 1 wherein said diffracted light has a polarization state produced by aligning the average relative permittivity tensor of said grating, said polarization state being one of linearly, elliptically or randomly polarized.
 14. The optical waveguide of claim 1 wherein non-diffracted has a polarization state produced by aligning the average relative permittivity tensor of said grating, said polarization state being one of linearly, elliptically or randomly polarized.
 15. The optical waveguide of claim 1 wherein said grating is one of a multiplexed set of gratings.
 16. The optical waveguide of claim 1 wherein said grating has a spatially variation of at least one of thickness, diffraction efficiency or k-vector direction.
 17. The optical waveguide of claim 1 wherein said grating comprise an array of selectively switchable elements.
 18. The optical waveguide of claim 1 wherein said diffracted light is transmitted through a TIR face of said waveguide.
 19. The optical waveguide of claim 1 further comprising at least one of a waveguide input coupler for inputting light through a face of said waveguide and directing it into said first propagation path, and a waveguide output coupler for outputting said diffracted light through a face of said waveguide, wherein each of said input and output couplers is one of a grating or prism.
 20. The optical waveguide of claim 1 wherein said input light is modulated with temporally-varying angularly-distributed information content. 